Project Summary Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by recurrent infections, hypogammaglobulinemia and autoimmunity. Although the etiology of CVID is largely unknown, deleterious mutations in the lipopolysaccharide-responsive beige-like anchor (LRBA) gene are associated with this syndrome. LRBA is a large intracellular adaptor protein (319 kD) localized to vesicular organelles including the endoplasmic reticulum, trans-Golgi complex, endocytic vesicles, and lysosomes. Although LRBA has been implicated in cell survival, proliferation, and autophagy, the precise function of LRBA in the endocytic pathway is unknown. LRBA has been recently shown to promote endosomal recycling of CTLA4, a potent inhibitory receptor expressed in T cells. LRBA-deficient patients expressed low levels of CTLA4 due to increased lysosomal degradation, and these patients exhibited marked improvement in inflammation and autoimmunity following treatment with CTLA4-Ig (abatacept) therapy. Importantly, abatacept did not improve immunoglobulin deficiency, suggesting a CTLA4-independent function of LRBA in humoral immunity. In support of this concept, B cells isolated from LRBA-deficient individuals failed to proliferate or secrete immunoglobulins in response to stimulation in vitro. Based on these findings, as well as data supporting LRBA in CTLA4 regulation, this application proposes that LRBA is a novel regulator of vesicular trafficking and promotes expression of key molecules required for the activation and function of B cells. Three specific aims are proposed to test this hypothesis in humans with loss-of-function mutations in LRBA. Specific Aim 1 will examine LRBA deficiency in B cell signal transduction and activation in vitro and in vivo, and Specific Aim 2 will use proteomic analyses to identify LRBA-regulated immunoreceptors required for B cell activation. Specific Aim 3 will identify the precise function of LRBA in the endocytic pathway by examining subcellular localization of LRBA, and the role of LRBA in synapse formation, vesicular transport and autophagy in B cells. Collectively, these studies will provide important insight into the immunoregulatory function of LRBA in B cells, which may lead to additional improvements in therapy for a severe form of immunodeficiency.